True, in other PB games new (and harder to kill) mobs respawn after you hit the next chapter.
In RIsen2 seems that once you clear an island and go away mobs respawn - but just once! You can't repeat that indefinetly and then earn yourself everything on 10. Respawns also appear in caves.

I've explored so far 5 areas, Caldera is the smallest one. Just a quarter of Tacarigua's size is Isle of Thieves, and other 3 areas (Sword Coast, Tacarigua and Antigua) felt like about the same size.